The contribution of animal fat to gunpowder (it is surprising that there is a connection!) is the nitrate content. Essentially the compost waste is turned into fertilizer, and from there to high explosives.

Nitrate is NO3. It is produced 100% from the air, by a process which is exothermic. So it’s (almost) free, at the margin, or would be if not for the need of pure input gasses.

A Haber-Bosch plant is a factory the size of a city that produces all the fertilizer or explosives you’d ever want, using nothing but air, water, and electricity as input.

We can confidently say that this particular shortage won’t be an issue anymore.